We have created a binary distribution of AVR GCC and associated tools for all Mac users. The package offers both, GCC version 3 and 4 because version 3 generates more efficient code. All software is compiled as universal binaries for Intel and PowerPC Macs.

It's a matter of personal preference. Newer versions are not always better.

I, personally, prefer gcc 3 because of the somewhat smaller code and because I'm used to its behavior (which warnings are on by default and so on).

On the other hand, new features are only added to gcc 4. If you want any of these, you probably want gcc 4. In some situations, gcc 4 generates even more efficient code than gcc 3 and there is hope that code generation will be improved in the future.